Dipole mode electromagnetic waveguides

ABSTRACT

A new form of waveguide is described which can support propagation of electromagnetic waves in the dipole mode. The waveguide is coaxial with inner and outer members having cylindrical surfaces separated by a dielectric. The longitudinal and transverse impedances of these surfaces and the dielectric material are such that propagation in the dipole mode is possible. Structures having the necessary surface impedances are described. Apparatus for launching the dipole mode and extracting power from it are also described.

United States Patent 1 Barlow Oct. 29, 1974 DIPOLE MODE ELECTROMAGNETIC WAVEGUIDES [75] Inventor: Harold Everard Monteagle Barlow,

Epsom, Surrey, England [73] Assignee: Natlonal Research Development Corp., London, England 22 Filed: July 24,1972

[21] Appl. No.: 274,619

[30] Foreign Application Priority Data Aug. 2, 1971 Great Britain 36203/71 Dec. 20, l97l Great Britain 58951/71 [52] US. Cl 333/95 S, 333/96 [51] Int. Cl. ll0lp 3/06, HOlp 3/l0 [58] Field of Search 333/95 S, 21 R, 96, 95 R [56] References Cited UNITED STATES PATENTS 4/1954 Allen 343/785 OTHER PUBLICATIONS Partch, .l. E. Dielectric Shielded G-Line MTT-l7, 5-1969, pp. 271-274. Barlow. H. E. M., Screened Surface Waves & Some Possible Applications, Proc. lEE Vol. ll2, 3-1965, pp. 477-482. Arora et al., M'I'T, Modes of Propagation in a Coaxial Waveguide with Lossless Reactive Guiding Surfaces," MTT-ZO, 3-72, pp. 210-214. Arora et al., R. 5., Modes of Propagation in a Parallel-Plate Waveguide with Lossless Reactive Surfaces, Radio Science 5, 5-1970, pp. 861-865.

Beam et al., "Shielded Dielectric-Rod Waveguides," AlEE Trans. Vol. 70, 1955, pp. 874-880.

Barlow, H. M., Radio Surface Waves," Oxford Press, 1962, pp. 60.

Kiely, D. 6., Dielectric Aerials, Methuen & Co. Ltd. 1953, pp. 26.

Kikuchi et al., "Theory of Dielectric Waveguides & Some Experiments at 50 KMC/Sec., Millimeter Waves," Proc. of Symposium, N.Y., N.Y., 1959, pp. 6l4, 63l, 634.

Primary Examiner-James W. Lawrence Assistant Examiner-Wm. H. Punter Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT A new form of waveguide is described which can support propagation of electromagnetic waves in the dipole mode. The waveguide is coaxial with inner and outer members having cylindrical surfaces separated by a dielectric. The longitudinal and transverse impedances of these surfaces and the dielectric material are such that propagation in the dipole mode is possible. Structures having the necessary surface impedances are described. Apparatus for launching the dipole mode and extracting power from it are also described.

30 Claims, 16 Drawing Figures PATENTEU HUT 29 I974 SHEEI 1 0F 3 FIG.

PATENTEDBBT 29 I574 sum 2 0F 3 FIGZb FIG. 20

The present invention relates to waveguides for the transmission of electromagnetic waves in a mode which has low attenuation and dispersion, and to apparatus for launching waves in this mode.

It is known that a single rod acting as a waveguide isolated in space, can support electric surface waves which, in the transverse plane, may be circularly symmetrical types designated E and H modes or dipole types described as EH and HE,, modes (see Radio Surface Waves by Barlow and Brown, Clarenden Press, Oxford I962). So far as the propagating medium is concerned none of these waves exhibits frequency cut-off and in principle they can all be transmitted along the outside of the guide at any part of the spectrum.

It is also known that the circularly symmetrical E and H modes can be screened and that the resulting coaxial structure, when comprising bare metal surfaces separated by a homogenous dielectric supporting the E configuration, becomes none other than the usual coaxial cable carrying the so-called T.E.M. mode.

According to a first aspect of the present invention there is provided apparatus for supporting electromagnetic waves, including a first elongated member, dielectric surrounding the first member to support propagation of a first surface wave along the interface between the first member and the dielectric, and a second elongated member, in which the second member surrounds the dielectric material to support the propagation of a second surface wave along the interface between the second member and the dielectric, and the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member are so chosen that the apparatus is able to support electromagnetic waves in the dipole mode with the first and second surface waves as constituents of the waves propagated in the dipole mode.

Preferably the outer surface of the first member and the inner surface of the second member are cylindrical and are coaxial.

In this specification and claims the term dipole mode" means any of the hybrid type EH, or HE, waves, where the letters E and H signify that there are electric and magnetic field components in the longitudinal direction and the subscript n periods of spatial distribution around the circumference. In the dipole mode the electric field is in the same direction on opposite sides of the first member along a diameter for n Also in this specification and claims the term dielectric" includes a vacuum.

The present invention springs from the discovery by the inventor that the dipole mode with boundary conditions suitably modified can also propagate when the known isolated waveguide is surrounded by a coaxial screen. He has also discovered that for appropriate surface impedances this mode exhibits no frequency cutoff.

The advantage of screened dipole mode propagation is that with a correctly devised waveguide attenuation can be very substantially less than the attenuation suffered by the more usual circularly symmetrical E mode,

if the dipole mode is the EH, mode or the HE, mode. Dispersion can also be considerably reduced.

Accordingly to a second aspect of the invention there is provided a transmission line for supporting an electromagnetic wave, including a first elongated member having a cylindrical outer surface, a second elongated member having a cylindrical inner surface, and dielectric separating the first and second members, the said surfaces being coaxial, and in which the equation Equation 1 where the symobols used in the equation have the meanings hereinafter defined, in satisfied when the apparatus supports electromagnetic waves of angular frequency w in the dipole mode with the number of periods of spatial distribution round the circumference of the first member equal to n. in this specification and claims the symbols used in the above equation designated Equation 1 have the following meanings:

7 the longitudinal propagation coefficient,

u the radial propagation coefficient,

r 4meZ /1ru S 0 r, the radius of the outer surface of the first member, r the radius of the inner surface of the second member, 2,, the longitudinal surface impedance of the outer surface of the first member, Z 0 1 the transverse surface impedance of the outer surface of the first member, Z n the longitudinal surface impedance of the inner surface of the second member, Z 0 the transverse surface impedance of the inner surface of the second member, s the permittivity of the dielectric material, a the permeability of the dielectric material, Sr =j(4/ ,.(u 1) n( 2) n( 2) .(u 1))l K, modified Bessel function of second kind and order n, modified Bessel junction of first kind and order "v S2 i( n( i) n th g) n l( 2) n( l))] a /1 'lUn ri il ni zl 11.0 am,- -1( 'i))] Equation i can be derived in the following way given by way of examples:

For a travelling wave on a cylindrical surface Maxwell's equations given:

where:

all the field quantities E and H are understood to contain, although not written down. the factor (2' 1 9 r. -Jn i E E and E the electric field vectors in the longitudinal, radial and circumferential directions, respectively;

H,, H, and H the magnetic field vectors in the longitudinal, radial and circumferential directions, respectively;

A and B are electric field amplitudes associated with the first and second members, respectively;

C and D are magnetic field amplitudes associated with the first and second members, respectively;

H,," Hankel function of the first type and order n;

Hf Hankel function of the second type and order h =ju =j(ajb); a transverse attenuation coefficient; b transverse phase change coefficient; 1 h v 0 +13 or longitudinal attenuation coefficient; B longitudinal phase change coefficient;

From equations 2 it can be shown that W H ll (roe/u) rl] 0| M/ it-6 '0".)l/l n U i) (D/C) H.."( i)l with F, corresponding to F and G to G, when the radius is r,,. lt is apparent therefore that two complementary surface waves occur each related to one of the supporting surfaces having radii r or r and, in any solution, both equations must be satisfied simultaneously. Equation I can now be derived from equations 3 and 4 when the conditions for a coaxial waveguide are applied. The inventor has also ound that if the inner and outer surfaces and the dielectric are such that:

then more simple relationships can be obtained.

If the condition of Equation 5 is utilized the impedances of the inner and the outer surfaces in the longitudinal direction are resistive and inductive, then the transverse impedances of these surfaces must be resistive and capacitive. On the other hand, Equation 5 is also satisfied if the longitudinal surface impedances are resistive and capacitive and the transverse surface impedances are resistive and inductive.

Thus according to a third aspect of the present invention there is provided a transmission line for supporting electromagnetic waves in the dipole mode, including a first elongated member inside a second elongated member, and dielectric material between the first and second members, the outer surface of the first member and the inner surface of the second member forming coaxial cylinders, wherein if the outer surface of the first member has transverse and longitudinal surface impedances designated Z, and Z respectively. and the inner surface of the second member has transverse and longitudinal surface impedance designated Z, and Z, respectively, when the equation 2,. Z. Z, Z Z is substantially fulfilled, where h is the c aracteristic impedance of the dielectric material.

Equation 5 is not necessarily satsified in meeting the requirements of Equation l. Indeed, with some antiresonant structures Z, can be much greater than 2 /2,

In this specification and claims the term transverse surface impedance" means the impedance at the surface to current transverse to the direction of propagation of waves in the waveguide, and longitudinal surface impedance means the impedance at the surface to current in the direction of propagation.

Advantageously, the dielectric has a relative dielectric constant in the range I to l0 and a relative permeability in the range I to 50.

Where the surface impedances of the first member are resistive and inductive in the longitudinal direction and resistive and capacitive in the transverse direction the first member may at high frequencies be a dielectric rod or a tube of dielectric material (advantageously having a ratio of inner radius to outer radius in the range 0.2 to 0.98), or an electrical conductor in the form of a conducting rod surrounded by a thick layer of dielectric material. Instead the first member may comprise longitudinal conductors which may be metal strips supported by a dielectric rod. Stranded insulated conductors with or without such support may be used instead.

Where the surface impedances of the inner surface of the second member are to be resistive and inductive in a longitudinal direction and resistive and capacitive in the transverse direction, the second member may include a number of parallel longitudinal conductors separated from one another. Again these conductors can be metal strips or stranded conductors.

According to a fourth aspect of the present invention there is provided apparatus for launching electromag netic waves in the dipole mode along a coaxial transmission line with an inner first member and an outer second member, including means for applying an alternating electric stress, at the frequency of a wave to be propagated, between pairs of opposite points on the inner surface of the outer member of a coaxial waveguide, the stress being in the same direction between the said opposite points over the dielectric between the first and second members on both sides of the first member.

For the HE, or EH, dipole modes, the said means for applying an electric stress may at high frequency be a rectangular waveguide capable of supporting the H mode, or a circular waveguide capable of supporting the H mode. The waveguide is terminated in a transverse conducting wall having an annular slot coupling to the coaxial waveguide, the slot being resonant at the frequency of propagation of the H or the H mode.

instead the end of the rectangular H or circular H waveguide may be left entirely open with the coaxial waveguide projecting slightly into, or at least adjacent to, the open end.

Certain embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which;

FIG. I shows pictorially the field pattern of the EH, dipole mode looking along a coaxial guide,

FIGS. 2a, b, c, d, e and f show various alternative forms for the inner of a coaxial waveguide for supporting the dipole mode,

FIGS. 30, b and c show various forms of the outer for a coaxial waveguide supporting the dipole mode,

FIGS. 40, b, c, d and e show various ways of launching electromagnetic waves in the dipole mode along a coaxial waveguide, and

FIG. 5 shows a coaxial waveguide with a composite dielectric.

In FIG. I a coaxial waveguide has an inner l0 and an outer ll (shown for convenience by means of a single line). The field pattern of the EH dipole mode is generally as shown but may differ in the relative strengths of the field as indicated by the distance between the lines of stress. The electric field in the transverse plane is indicated by solid lines designated E and the magnetic in the transverse lane is indicated by dotted lines designated H. Since both the electric and the magnetic fields also have longitudinal components the lines fully representing the two fields would have components at right angles to the plane of the Figure. It will be seen that along the vertical diameter of the waveguide as shown in FIG. I the electric field is in the same direction on both sides of the inner It]. This is in contrast to the circularly symmetrical E and H modes where the electric field is in opposite directions on either side of the inner 10. Another difference between the circularly symmetrical modes and the dipole modes is that in the longitudinal direction the circularly symmetrical modes have only electric or magnetic components but the dipole modes have both electric and magnetic components.

In higher order dipole modes that is where n l there are n regions in which the dipole field is repeated. For example where n 2, the field which occurs in one semi-circumference of FIG. 1 occurs in each quadrant. A magnetic field is associated with each such diameter in the way shown in outline in FIG. 1 for n l.

The dipole mode may, for simplicity of approach, be regarded as two separate surface waves one of which propagates along the surface of the inner and the other propagates along the surface of the outer. Nevertheless, the two waves have to accommodate themselves jointly in the dielectric space, and are superimposed one on the other.

The reason for the comparatively low attenuation of the dipole mode can be quantitatively explained by noting that the electric and magnetic fields are much more uniformly distributed over the dielectric space than in the circularly symmetrical mode where the strength of the electric and magnetic fields falls as the inverse of the distance from the inner. Hence in the circularly symmetrical modes most of the power is transmitted in close proximity to the inner and therefore the loss of the inner supporting surface and the dielectric material is greater.

Moreover, the comparatively small value of radial propagation coefficient u which is characteristic of the lowest-order dipole-mode (EH, or HE,) reduces dispersion. Thus, the phase-change coefficient 3 for this guided wave, is not only very close to that of the corresponding free-space wave, but its variation with frequency becomes almost linear. Consequently, both in the reduction of losses and in the avoidance of phasedistortion for telecommunication purposes, the screened dipole-mode has much to recommend it.

Attenuation can be reduced further if the outer surface wave is made dominant by making the inner surface of the outer better able to support a surface wave.

Although circularly symmetrical E and H modes in a coaxial guide are always clearly distinguishable, there is no similar clear-cut difference between the EH, and HE,, screened dipole modes both of which, as hybrids, contain all field components. Thus, the EH. and HE,, waves can only be distinguished by the relative strengths of the E and H field components in the direction of propagation. When the longitudinal E is relatively the stronger we speak of an EH mode and when the longitudinal H is dominant we have an HE mode, such differences as arise in practice depending upon the nature of the supporting surfaces.

The dipole mode propagates without frequency cutoff, so that in order to prevent the propagation of modes which depend on a relationship between wavelength and transverse guide dimensions, the dimensions of the coaxial guide are preferably so chosen that such modes are cut off.

In devising a waveguide to support the EH, or HE dipole modes, the equations can be simplified by satisfying the conditions of Equation 5 which leads to a condition that if the inner 10 has a resistive and inductive surface impedance in a longitudinal direction, it must have a resistive and capacitive surface impedance in the transverse direction, and the outer must have similar surface impedances. It is known (see the above mentioned book Radio Surface Waves" by Barlow HM. and Brown J.) that a dielectric rod has the required surface impedances so that, as shown in H0. 2a, the inner 10 may be a solid dielectric rod. For example a rod made of one or more of the following plastics materials may be used: polythene, polystyrene, polyethylene, P.T.F.E. or similar plastics material. FIGS. 2b and c show other types of inner with the same kind of impedances. in FIG. 2b a dielectric tube is used and in FIG. 2c a metal rod 12 surrounded by a thick dielectric coating 13 is used. The plastics materials mentioned above may, for example, be used. Where the metal rod surrounded by dielectric is used, the following paper shows how the required impedances can be obtained: Theory of dielectric waveguides and some experiments at K.Mc/sec." by Kikuchi H. and Yamashita 5, Proceedings of the Symposium on Millimeter Waves, New York I959, pages 6l9 638, published by the Polytechnic Press. In general the ratio of the radii of the metal rod and the outer surface of the dielectric can have any value but preferably the ratio should not be near one for low frequencies.

Capacitance in the transverse direction can be obtained by using parallel longitudinal conductors for example copper or brass separated from one another, such as the metal strips Id of FIG. 2d. With this arrangement, in order for the capacitance to appear uniformly distributed there must be at least three metal strips 14 within one wavelength as measured circumferentially round the inner at the frequency of the sig nal to be propagated by the guide. For convenience the metal strips may be mounted on a dielectric substrate 15 and this arrangement is particularly appropriate at low frequencies. It will be seen from Equation I that in principle dipole waves can be propagated in a coaxial waveguide at any frequency.

Another way of achieving capacitance in the transverse direction is to use parallel longitudinal grooves 16 in a metal rod, for example copper or brass, as shownin FIG. 2e. When a wave travels down such a groove it is reflected at the end and returns to the surface. As is shown, if the groove is of the correct depth, the resulting addition of the incident and reflected waves is equivalent to a capacitance at the entrance to the groove. If the groove is made very narrow compared to its depth, then the depth itself is not important in the upper range of frequencies since the field in it can be practically evanescent giving a capacitive impedance at the groove entrance. Again there must be at least three grooves per wavelength as measured circumferentially round the rod in order to give the efi'ect of distributed capacitance.

In FIG. Zfan arrangement is shown for the inner 10 which is in some ways the inverse of the arrangement of FIG. 2e. A longitudinal metal core 17 has radial fins l8 which extend parallel to one another in the longitudinal direction. The fins are embedded in a cylinder of dielectric l9 and are of such a length, and of such a number per wavelength that the surface presents 3 capacitive impedance in the transverse direction. Again the dielectric may be the above mentioned plastics materials. This type of structure is discussed in the above mentioned book Radio Surface Waves" at pages 57 to 59.

With regard to the outer, longitudinal inductance and transverse capacitance can be obtained in the same ways as used in FlGS. 2d, e andffor the inner. For example the outer may comprise a number of parallel longitudinal metal strips 2! (FIG. 30) mounted on the inside ofa dielectric tube 22. Again there must be sufficient strips to present what is in effect a distributive capacitancc at the frequency of propagation. In another arrangement (not shown) the outer may include a num ber of longitudinal metal wires embedded just below the inner surface of a tube of plastics material. The

outer surface of the tube may be covered with a braided screen of copper. If the operating frequency is 3Gl'l the wires should be copper wires of small diameter, for example 42 to 47 S.W.G. The wires are spaced about 1 cm apart round the circumference of the inner surface but just below the surface. A suitable plastics material is polythene and the thickness of the tube wall can advantageously be approximately L57 cms. or a quarter of a wavelength at 3GH,. In this way penetration of the field into the tube wall is minimized. The wires can be skewed into a shallow helix provided the circumferential spacing is maintained continually. A similar structure of copper wires in a rod of plastics material may be used for the inner.

In FIG. 3b a metal cylinder with parallel longitudinal grooves 23 is used for the outer H, the grooves being of the correct depth to present a capacitance at their entrance or arranged to support an evanescent field so that where the skin depth is small the grooves in effect form gaps in the rods surface. As an alternative the outer 11 may be as shown in H6. 3c where a metal cylinder has parallel longitudinal fins 24 radially arranged on its inside and the fins are embedded in a cylindrical layer of dielectric material 25. For the outer where a dielectric is used it may be of the above mentioned plastics materials, and copper or brass are suitable metals where a conductor is required in the outer.

The ratio of the radii of the outer to the inner surfaces may have any chosen value providing Equation 1 is satisfied but a convenient practical limit is 20: l.

in general, for high power transmission a strong longitudinal current is required and in these circumstances it is desirable to make the surface impedances of the inner and outer in the transverse direction small with correspondingly large values of longitudinal inner and outer surface impedances. The longitudinal grooved structure has merits in this case.

Although the dielectric between the inner and outer members will usually be air, any material with suitable dielectric properties may be used, and the dielectric may be a composite dielectric as shown in FIG. 5. For simplicity a rod inner 10 and tubular outer II are shown. Dielectric spacers 40 are evenly distributed along the inner with air spaces between them.

A first example of the dimensions, impedances and materials used in a waveguide, which will support the dipole mode of order n 1 will now be given:

Frequency: 3GH Inner: Dielectric rod of diameter 0.3 cms., dielectric constant 2.5, Tan delta 1 X 10, (one of the above mentioned plastics materials is suitable) giving:

2,, 4.42 j 2.64 x 10 ohms.

Outer: Brass strips on the inner surface of a plastics tube (again one of the above plastics materials can be used), diameter 2.5 cms.

2., (2.25 X 10 2.25 X l0), the condition 2 2 Zf/Z being satisfied for air as the dielectric between inner and outer, and Z, for brass. With these values a 3.67 X 10 Nepers/metre [3 63.28 Radians/metre a 7. l6 Nepers/metre 3.24 X 10 Radians/metre.

In another example with operation at 3GH,:

Inner: Brass strips with a thin coating of one of the above mentioned dielectric materials, diameter L57 X lO" metres 2;, L! X j8.6 X l0 ohms Outer: Brass strips, diameter 25 X metres Z 2.26 X 10 +j (2.26 X l0) ohms The inner and outer are separated by air and n l, givmg a 3.33 X 10 Nepers/metre B 62.8 Radians/metre The corresponding circularly symmetrical E mode in a guide of the same dimensions would give an attenuation of l0.l X l0 Nepers/metre.

At 50 Hz this last example gives:

a 6.07 X 10 Nepers/metre The attenuation of the usual T.E.M. mode in these circumstances would be 13.l3 X 10" Nepers/metre.

In order to launch the dipole mode the electric field must be applied in the same direction across the whole diameter of the outer from one to the other. in order to obtain such a field distribution a rectangular waveguide supporting the H mode may be used. Alternatively a circular waveguide supporting the H mode may be used. FIG. 4a and 4b shows these types of waveguide and the electric field therein designated by E.

Either waveguide may be coupled to the coaxial line which will support the dipole mode in the way shown in section in H6. 4c. Here the coaxial waveguide has an inner l0 and an outer ll and the rectangular or circular waveguide 26 has a terminating transverse wall 28 in which a resonant annular slot 29 is cut. Such coupling slots are known and are described in Microwave Design Data", by Marine, Constable & Co., Dover Publishers. in the counterpart of the annular slot, a conducting ring is placed in the open end of the waveguide, its circumferential dimension being resonant at the frequency of propagation. With such an arrangement a wave propagating in the waveguide is reflected back along the guide. Thus where an annular slot is provided instead of the conductive ring coupling is obtained to the coaxial waveguide.

The arrangement of H0. 40, b and c is particularly suitable for coupling into a resonant cavity. For ordinary transmission the end of the rectangular H or circular H waveguide may conveniently be left open to the dipole mode guide. For example the wall 28 may be omitted and the inner 10 may project into the rectangular or circular waveguide.

In the arrangement of FIG. 4c and the arrangement in which the wall 28 is omitted, the end 30 of the inner [0 may be tapered if necessary, forming where the end wall is present a transition between the inner circumference of the annulus and the required diameter of the inner 10.

If the outer has the form shown in FIG. 3a the launching arrangement shown in FIG. 40' may be used. Here two of the metal strips 2] have extensions 32 which form a two wire line on which the T.E.M. mode can be impressed. The coaxial waveguide extends in the direction of the arrow 33 and the inner 10 is tapered from its correct dimension to a point 34 approximately in the plane where the strips 21 terminate and the extensions 32 begin.

Another way of launching the dipole mode is to use a two wire line 35 supporting the T.E.M. mode (see HO. 4e). The coaxial waveguide with its tapered inner l0 and its outer ll projects between the conductors of the line 35, and an adjustable short circuit 36 is provided between the outer 11 and the conductors of the line 35. By movement of the short circuit 36 the circular waveguide can be matched to the coaxial waveguide.

ln order to prevent waveguide modes dependent upon transverse resonance from propagating, the dimensions of the coaxial waveguide are made such that it is below cut-off for these unwanted modes. If necessary a transition may be used in accommodating the coaxial waveguide to another type of waveguide of larger dimensions used for launching purposes.

Any of the arrangements used to launch the dipole mode on a coaxial transmission line may be used to extract energy from a line carrying this mode. This is a known principle which as would be expected also applies to the dipole mode.

Higher order dipole modes can be launched using waveguides supporting modes having an appropriate field pattern.

It will be understood that although various forms of coaxial waveguide, and inner and outer, various launching arrangements have been described, the invention is not limited to these specifically described examples and in fact any coaxial waveguide which will support the dipole mode can be used in carrying out the invention, as can any launching arrangement which will provide the necessary field to launch the dipole mode on a coaxial waveguide.

l claim:

1. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, and

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse and longitudinal surface impedances of the second member being different, and

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constitutents of the waves propagated in said dipole mode.

2. A transmission line according to claim 1 wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders.

3. A transmission line according to claim 2 wherein the dielectric is a composite dielectric including solid dielectric spacers between the first and second members spaced longitudinally from one another. and gas between the spacers.

4. A transmission line according to claim 2 wherein the dielectric includes air.

5. A transmission line according to claim 2 wherein the first member is a rod of dielectric material having a relative dielectric constant in the range l to 10, and a relative permeability in the range 1 to 50.

6. A transmission line according to claim 2 wherein the first member is a tube of dielectric material having a ratio of inner radius to outer radius in the range 0.2 to 0.98, and the material of the tube having a relative dielectric constant in the range I to 10, and a relative permeability in the range I to 50.

7. A transmission line for supporting an electromagnetic wave, including a first elongated member having a cylindrical outer surface, a second elongated member having a cylindrical inner surface, and dielectric separating the first and second members, the said surfaces being coaxial, and in which the equation where the symbols used in the equation have the meanings hereinbefore defined, is satisfied when the apparatus supports electromagnetic waves of angular frequency w in the dipole mode with the number of periods of spatial distribution round the circumference of the first member equal to its 8. A transmission line according to claim 7 wherein the first member is a rod of dielectric material having a relative dielectric constant in the range one to ten, and a relative permeability in the range one to fifty and the inner surface of the second member is made up of a plurality of elongated conductors spaced one from another.

9. A transmission line according to claim 7 wherein' the first member is a tube of dielectric material having a ratio of inner radius to outer radius in the range 0.2 to 0.98, and the material of the tube having a relative dielectric constant in the range one to ten, and a relative permeability in the range I to 50 and the inner surface of the second member is made up ofa plurality of elongated conductors spaced one from another.

It). A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric.

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constitutents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member from coaxial cylinders, and

wherein the outer surface of the first member and the inner surface ofthe second member each have longitudinal surface impedances which are resistive and inductive, and transverse surface impedances which are resistive and capacitive,

H. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric,

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode ith said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and

wherein the outer surface of the first member is made up of a plurality of elongated conductors spaced one from another.

l2. A transmission line according to claim I] wherein the elongated conductors are thin metal strips mounted parallel to one another on an insulating rod.

13. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric,

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and

wherein the first member is a rod of conducting material having at least three grooves within the wavelength measured circumferentially round the rod for the highest frequency of electromagnetic waves to be propagated, the grooves being radial and extending longitudinally with their apertures in the outer surface of the rod parallel, and of a depth such that at their said apertures a capacitive impedance is presented over at least part of the said frequency range.

14. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric.

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and

wherein the first member includes a rod of conducting material having at least three conducting fins within the wavelength measured circumferentially round the rod for the highest frequency of electromagnetic waves to be propagated, the fins extending radially, with their junctions with the rod extending longitudinally parallel with one another, the fins being embedded in dielectric material having a cylindrical outer surface forming the outer surface of the first member, and the fins being of such a depth in the radial direction that the transverse impedance at the said outer surface is capacitive.

15. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric,

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface ofthe first member and the inner surface of the second member form coaxial cylinders, and

wherein the inner surface of the second member is made up of a plurality of elongated conductors spaced one from another.

[6. A transmission line according to claim 15 wherein the elongated conductors are thin metal strips mounted parallel to one another on the inner surface of an insulating cylinder.

17. A transmission line according to claim 16 in combination with apparatus for launching electromagnetic waves in the dipole mode along the transmission line, wherein two diametrically opposite strips of the second member of the transmission line extend from a transverse plane where the other strips and the first member terminate to form a two wire line suitable for supporting the transverse electric mode, and the apparatus ineludes means for applying an alternating electric stress, at the frequency of a wave to be propagated, between the said diametrically opposite strips.

18. A transmission line for supporting electromagnetic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric,

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and

wherein the second member is a cylinder of conducting material having at least three grooves within the wavelength in the inner surface measured circumferentially round the said inner surface for the highest frequency of electromagnetic waves to be propagated, the grooves being radial and extending longitudinally with their apertures in the inner surface of the cylinder being parallel, and of a depth such that at their said apertures a capacitive impedance is presented over the frequency range of waves to be propagated.

19. A transmission line for supporting electromag netic waves, including:

a first elongated member,

dielectric surrounding the first member and adapted to support propagation ofa first surface wave along the interface between the first member and the dielectric,

a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric,

the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode,

wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and

wherein the second member includes a cylinder of conducting material having at least three conducting fins within the wavelength measured circumferentially round the said inner surface for the highest frequency of electromagnetic waves to be propagated. the fins extending radially inwards, with their junctions with the cylinder extending longitudinally parallel with one another. the fins being embedded in dielectric material having a cylindrical inner surface forming the inner surface of the second member, and the fins being of such a length that the said inner surface is capacitive.

20. A method of propagating electromagnetic waves wherein the waves are propagated as first and second dipole mode surface waves along coaxial cylindrical surfaces of first and second members, respectively, the transverse and longitudinal surface impedance of the second member being different and the surface waves together constituting the dipole mode in dielectric between the coaxial cylindrical surfaces.

21. Apparatus for launching electromagnetic waves in the dipole mode along a coaxial transmission line, including a coaxial transmission line in combination with means or inducing an alternating magnetic field in the dielectric material between inner and outer members of a coaxial waveguide with the transverse and iongitudinal surface impedances of the outer member being different and thus adapted to support electromagnetic waves in the dipole mode with surface waves propagating along the interface of each member as constituents of waves propagated in the dipole mode. the induced magnetic field forming flux loops transverse to the waveguide which are not centered on the inner member.

22. Apparatus for launching electromagnetic waves in the dipole mode along a coaxial transmission line with an inner first member and an outer second member, including means for applying an alternating electric stress, at the frequency of a wave to be propagated, between sectors in at least one pair of sectors opposingly disposed on the inner surface of the outer member of a coaxial waveguide, the stress across the waveguide between the sectors in each pair being in the same direction in the dielectric on both sides of the first member.

23. Apparatus according to claim 22 for extracting energy from a coaxial transmission line adapted to support electromagnetic waves in the dipole mode.

24. Apparatus according to claim 22 for launching electromagnetic waves in the lowest order dipole in which there is one said pair only.

25. Apparatus according to claim 24 including a rectangular waveguide capable of supporting electromagnetic waves in the H mode over a range of frequen cies, the rectangular waveguide having a transverse wall at one end with an annular aperture therein, the

aperture being of resonant circumferential length at the predetermined frequency, and a coaxial waveguide constructed to support the dipole mode fixed to the transverse wall on the other side from the rectangular waveguide, with the inner member of the coaxial waveguide adjacent to that part of the transverse wall inside the annulus.

26. Apparatus according to claim 24 including a rectangular waveguide, capable of supporting electromagnetic waves in the H mode over a range of frequencies, joined end to end with a coaxial waveguide constructed to support the dipole mode, the outer member of the coaxial waveguide being joined to the walls of the rectangular waveguide and the inner member of the coaxial waveguide being adajcent to the end of the rectangular waveguide.

27. Apparatus according to claim 24 including a circular waveguide, capable of supporting electromagnetic waves in the H mode over a range of frequencies, joined end to end with a coaxial waveguide constructed to support the dipole mode, the outer member of the coaxial waveguide being joined to the wall of the circular waveguide and the inner member of the coaxial waveguide adjacent to the end of the circular waveguide.

28. Apparatus according to claim 24 including a circular waveguide capable of supporting electromagnetic waves in the H mode at a predetermined frequency. the circular waveguide having a transverse wall at one end with an annular aperture therein, the aperture being of resonant circumferential length at the predetermined frequency, and a coaxial waveguide constructed to support the dipole mode fixed to the transverse wall on the other side from the circular waveguide, with the inner member of the coaxial waveguide adjacent to that part of the transverse wall inside the annulus.

29. Apparatus according to claim 24 including a twin conductor transmission line capable of supporting T.E.M, waves, a coaxial transmission line capable of supporting waves in the dipole mode, with one end projecting between the twin conductors and an inner member which tapers towards the said end, and matching means forming a short circuit between the conductors of the twin conductor line and the outer of the coaxial transmission line, the position of the matching means being adjustable longitudinally.

30. Apparatus according to claim 24 for extracting energy from a coaxial transmission line adapted to support electromagnetic waves in the dipole mode.

a x: a: r 

1. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, and a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse and longitudinal surface impedances of the second member being different, and the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constitutents of the waves propagated in said dipole mode.
 2. A transmission line according to claim 1 wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders.
 3. A transmission line according to claim 2 wherein the dielectric is a composite dielectric including solid dielectric spacers between the first and second members spaced longitudinally from one another, and Gas between the spacers.
 4. A transmission line according to claim 2 wherein the dielectric includes air.
 5. A transmission line according to claim 2 wherein the first member is a rod of dielectric material having a relative dielectric constant in the range 1 to 10, and a relative permeability in the range 1 to
 50. 6. A transmission line according to claim 2 wherein the first member is a tube of dielectric material having a ratio of inner radius to outer radius in the range 0.2 to 0.98, and the material of the tube having a relative dielectric constant in the range 1 to 10, and a relative permeability in the range 1 to
 50. 7. A transmission line for supporting an electromagnetic wave, including a first elongated member having a cylindrical outer surface, a second elongated member having a cylindrical inner surface, and dielectric separating the first and second members, the said surfaces being coaxial, and in which the equation
 8. A transmission line according to claim 7 wherein the first member is a rod of dielectric material having a relative dielectric constant in the range one to ten, and a relative permeability in the range one to fifty and the inner surface of the second member is made up of a plurality of elongated conductors spaced one from another.
 9. A transmission line according to claim 7 wherein the first member is a tube of dielectric material having a ratio of inner radius to outer radius in the range 0.2 to 0.98, and the material of the tube having a relative dielectric constant in the range one to ten, and a relative permeability in the range 1 to 50 and the inner surface of the second member is made up of a plurality of elongated conductors spaced one from another.
 10. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constitutents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member from coaxial cylinders, and wherein the outer surface of the first member and the inner surface of the second member each have longitudinal surface impedances which are resistive and inductive, and transverse surface impedances which are resistive and capacitive.
 11. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen To cooperate in causing the transmission line to support electromagnetic waves in the dipole mode ith said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and wherein the outer surface of the first member is made up of a plurality of elongated conductors spaced one from another.
 12. A transmission line according to claim 11 wherein the elongated conductors are thin metal strips mounted parallel to one another on an insulating rod.
 13. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and wherein the first member is a rod of conducting material having at least three grooves within the wavelength measured circumferentially round the rod for the highest frequency of electromagnetic waves to be propagated, the grooves being radial and extending longitudinally with their apertures in the outer surface of the rod parallel, and of a depth such that at their said apertures a capacitive impedance is presented over at least part of the said frequency range.
 14. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and wherein the first member includes a rod of conducting material having at least three conducting fins within the wavelength measured circumferentially round the rod for the highest frequency of electromagnetic waves to be propagated, the fins extending radially, with their junctions with the rod extending longitudinally parallel with one another, the fins being embedded in dielectric material having a cylindrical outer surface forming the outer surface of the first member, and the fins being of such a depth in the radial direction that the transverse impedance at the said outer surface is capacitive.
 15. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surRounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and wherein the inner surface of the second member is made up of a plurality of elongated conductors spaced one from another.
 16. A transmission line according to claim 15 wherein the elongated conductors are thin metal strips mounted parallel to one another on the inner surface of an insulating cylinder.
 17. A transmission line according to claim 16 in combination with apparatus for launching electromagnetic waves in the dipole mode along the transmission line, wherein two diametrically opposite strips of the second member of the transmission line extend from a transverse plane where the other strips and the first member terminate to form a two wire line suitable for supporting the transverse electric mode, and the apparatus includes means for applying an alternating electric stress, at the frequency of a wave to be propagated, between the said diametrically opposite strips.
 18. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner surface of the second member form coaxial cylinders, and wherein the second member is a cylinder of conducting material having at least three grooves within the wavelength in the inner surface measured circumferentially round the said inner surface for the highest frequency of electromagnetic waves to be propagated, the grooves being radial and extending longitudinally with their apertures in the inner surface of the cylinder being parallel, and of a depth such that at their said apertures a capacitive impedance is presented over the frequency range of waves to be propagated.
 19. A transmission line for supporting electromagnetic waves, including: a first elongated member, dielectric surrounding the first member and adapted to support propagation of a first surface wave along the interface between the first member and the dielectric, a second elongated member, in which the second member surrounds the said dielectric and is adapted to support the propagation of a second surface wave along the interface between the second member and the dielectric, the transverse dimensions of the first and second members and the impedances of the outer surface of the first member and the inner surface of the second member being so chosen to cooperate in causing the transmission line to support electromagnetic waves in the dipole mode with said first and second surface waves being constituents of the waves propagated in said dipole mode, wherein the outer surface of the first member and the inner suRface of the second member form coaxial cylinders, and wherein the second member includes a cylinder of conducting material having at least three conducting fins within the wavelength measured circumferentially round the said inner surface for the highest frequency of electromagnetic waves to be propagated, the fins extending radially inwards, with their junctions with the cylinder extending longitudinally parallel with one another, the fins being embedded in dielectric material having a cylindrical inner surface forming the inner surface of the second member, and the fins being of such a length that the said inner surface is capacitive.
 20. A method of propagating electromagnetic waves wherein the waves are propagated as first and second dipole mode surface waves along coaxial cylindrical surfaces of first and second members, respectively, the transverse and longitudinal surface impedance of the second member being different and the surface waves together constituting the dipole mode in dielectric between the coaxial cylindrical surfaces.
 21. Apparatus for launching electromagnetic waves in the dipole mode along a coaxial transmission line, including a coaxial transmission line in combination with means or inducing an alternating magnetic field in the dielectric material between inner and outer members of a coaxial waveguide with the transverse and longitudinal surface impedances of the outer member being different and thus adapted to support electromagnetic waves in the dipole mode with surface waves propagating along the interface of each member as constituents of waves propagated in the dipole mode, the induced magnetic field forming flux loops transverse to the waveguide which are not centered on the inner member.
 22. Apparatus for launching electromagnetic waves in the dipole mode along a coaxial transmission line with an inner first member and an outer second member, including means for applying an alternating electric stress, at the frequency of a wave to be propagated, between sectors in at least one pair of sectors opposingly disposed on the inner surface of the outer member of a coaxial waveguide, the stress across the waveguide between the sectors in each pair being in the same direction in the dielectric on both sides of the first member.
 23. Apparatus according to claim 22 for extracting energy from a coaxial transmission line adapted to support electromagnetic waves in the dipole mode.
 24. Apparatus according to claim 22 for launching electromagnetic waves in the lowest order dipole in which there is one said pair only.
 25. Apparatus according to claim 24 including a rectangular waveguide capable of supporting electromagnetic waves in the H01 mode over a range of frequencies, the rectangular waveguide having a transverse wall at one end with an annular aperture therein, the aperture being of resonant circumferential length at the predetermined frequency, and a coaxial waveguide constructed to support the dipole mode fixed to the transverse wall on the other side from the rectangular waveguide, with the inner member of the coaxial waveguide adjacent to that part of the transverse wall inside the annulus.
 26. Apparatus according to claim 24 including a rectangular waveguide, capable of supporting electromagnetic waves in the H01 mode over a range of frequencies, joined end to end with a coaxial waveguide constructed to support the dipole mode, the outer member of the coaxial waveguide being joined to the walls of the rectangular waveguide and the inner member of the coaxial waveguide being adajcent to the end of the rectangular waveguide.
 27. Apparatus according to claim 24 including a circular waveguide, capable of supporting electromagnetic waves in the H11 mode over a range of frequencies, joined end to end with a coaxial waveguide constructed to support the dipole mode, the outer member of the coaxial waveguide being joined to the wall of the circular waveguide and the inner member of the coaxiaL waveguide adjacent to the end of the circular waveguide.
 28. Apparatus according to claim 24 including a circular waveguide capable of supporting electromagnetic waves in the H11 mode at a predetermined frequency, the circular waveguide having a transverse wall at one end with an annular aperture therein, the aperture being of resonant circumferential length at the predetermined frequency, and a coaxial waveguide constructed to support the dipole mode fixed to the transverse wall on the other side from the circular waveguide, with the inner member of the coaxial waveguide adjacent to that part of the transverse wall inside the annulus.
 29. Apparatus according to claim 24 including a twin conductor transmission line capable of supporting T.E.M. waves, a coaxial transmission line capable of supporting waves in the dipole mode, with one end projecting between the twin conductors and an inner member which tapers towards the said end, and matching means forming a short circuit between the conductors of the twin conductor line and the outer of the coaxial transmission line, the position of the matching means being adjustable longitudinally.
 30. Apparatus according to claim 24 for extracting energy from a coaxial transmission line adapted to support electromagnetic waves in the dipole mode. 